A dike 33 km long is being constructed to reclaim the Saemangeum coastal zone, located mid-west of Korean peninsula. We attempted to evaluate change in tides of the Yellow Sea by the dike construction using a 3-dimensional numerical model (modified Princeton Ocean Model). An wetting-drying condition is employed to reproduce tidal flat area and a semi-implicit scheme is applied for the external mode calculation. Using the semi-implicit scheme, we can increase numerical stability and efficiency of the W-D condition. We considered three different cases of coastline (before, after and during the dike construction) and two initial and open-boundary conditions for summer and winter. Fine grid Saemangeum area model is nested from coarse grid models. Deformation of coastline in Saemangeum area, located on the entrance of tidal energy propagation into the Yellow Sea, influences on the tidal system of Yellow Sea and can cause change of marine environment.